An erbium doped fiber amplifier (EDFA) was used in a submarine cable optical repeater for the first time in a submarine cable project in 1996, which ushered in a new era of the submarine cable optical relay technology. Compared with a traditional regeneration-type optical repeater, design is significantly simplified and system reliability is improved. However, the introduction of the erbium doped fiber amplifier also brings a new problem, that is, the decrease of an optical signal-to-noise ratio (OSNR) caused by accumulation of spontaneous emission noise. A submarine cable system with repeaters usually uses relay distances of an equal span, and a gain of the optical repeater exactly compensates an optical fiber loss of a span. An optical signal-to-noise ratio of receiving-end signals can be obtained through calculation by using the following formula:
      OSNR    ⁡          (      dB      )        =      58    -          10      ⁢              log        ⁡                  (                                                    ∑                N                                            i                =                1                                      ⁢                                          NF                i                                            P                i                in                                              )                    where NFi denotes a noise factor (a linear value) of the ith optical repeater, Piin denotes single-wavelength input power (a linear value) of the ith optical repeater, and N is a quantity of the optical repeaters. It can be seen from the foregoing formula that an OSNR of a signal light is determined by a noise factor of an optical repeater at each level and input power of a signal light in an optical repeater at each level.
According to the foregoing formula, it can be learned that: when all optical repeaters are normal, noise factors and single-wavelength input power of all optical repeaters are the same, and contributions of all optical repeaters to accumulation of a system OSNR are also the same; if one optical repeater in the system fails and as a result, its output power is decreased, input power of a downstream neighboring optical repeater is decreased and more spontaneous emission noise is introduced and accumulated; and if pump power of one optical repeater in the system is completely lost, an erbium doped fiber (EDF) in the optical repeater also produces a significant absorption loss to signals, which significantly decreases input power of a downstream neighboring optical repeater and as a result, the system OSNR is degraded abruptly. Generally, two pumps are used for redundancy backup in a repeater, to maintain high enough signal input power of the optical repeater, restrain excessive accumulation of spontaneous emission noise, and avoid severe degradation of an optical signal-to-noise ratio. Enough high output power can be maintained even when one pump fails, so that input power of a downstream optical repeater is not significantly decreased and an optical signal-to-noise ratio is not significantly degraded. In the industry, four pumps are also used for redundancy backup in a repeater. In this case, a system service is not interrupted even when three pumps fail. However, when all pumps in a fiber pair fail or a power supply unit of the optical repeater fails, the foregoing solutions may cause rapid degradation of the system OSNR and even service interruption.
In another aspect, as signals carried by a submarine cable develop towards a signal transmission rate of above 100 Gb/s, a higher-order modulation format, such as 8 quadrature amplitude modulation (8QAM) and 16 quadrature amplitude modulation (16QAM), requires a higher optical signal-to-noise ratio, which limits a system transmission distance. A distributed Raman amplification technology can provide a lower noise factor to improve the system OSNR; however, productization of an underwater optical repeater is difficult due to restriction by elements, such as, high power consumption, and that an operating point close to a linear area is disadvantageous to fault tolerance. As a compromise solution, hybrid amplification implemented by a reverse distributed Raman amplifier and an erbium doped fiber amplifier (EDFA) helps to reduce a noise factor without increasing much power consumption, and is a technical development trend of an underwater optical repeater. However, an optical repeater with hybrid amplification of a reverse distributed Raman amplifier and an EDFA also has a problem of technical deficiencies as those in the optical repeater based on the EDFA technology. When an EDFA pump fails, an EDF may produce an absorption loss to a signal, causing degradation of an optical signal-to-noise ratio of a service signal.
Generally, for underwater breakdown maintenance of a submarine cable communications system, a dedicated submarine cable maintenance ship needs to be dispatched, a maintenance period is about 2 weeks on average, and most of the time is spent on spare part transportation and sailing. An economic loss caused by service interruption during a system breakdown period is huge; therefore, a submarine cable optical repeater needs to be highly reliable. However, when an EDFA pump in the optical repeater fails, the EDF may produce an absorption loss to a signal, which causes abrupt degradation of an OSNR of a service signal and even service interruption, and therefore, reliability of the optical repeater is low.